Well, wush is kind of special because it cannot proc for I believe 11 seconds after you got a proc, because that would screw over the stacking part of the proc.
BUT, even though during those 11 seconds the chance to proc is equal to 0, the time_since_last_chance_to_proc keeps incrementing and will equal to 10 at the end of the proc as if you did no dmg at all for 10 seconds (it's not gonna be 11 because it's capped at 10).
So basically, the first time you deal damage after the proc faded, you are gonna have 1.21 * (10/60) = 20.16% chance to proc. This is just the very first instance of damage you deal. After that, the proc chance is back to normal.
You can get lucky and sometimes have proc after proc, but it's not gonna happen that often (1/5 of the times). Proc after proc doesn't really benefit destro anyway, since you burn your embers during the first proc.